How big can/should wall colliders be?

Following my previous question, I now have my wall collider building tool and built many of them:

Bird-view, about 17 walls selected, on the right, one is selected:

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Here, wall and barriers form a collider so the car will hit the barrier:

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Here you can climb on that little slope:

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Having either a single, large collider or tons of small ones doesn’t sound good to me.

What’s a good metric in deciding how many (and how big) colliders should there be?

Also, should they be as simple as possible, e.g. less points for straight lines?